1. Field of the Invention
The present invention relates to a radar device and a radar method, and relates specifically to a radar device and a radar method for accurate detection of an object in front of the radar.
2. Description of the Related Art
Monopulse type radars mounted to a vehicle can be used to avoid a collision between vehicle and other vehicle, with the radar detecting the other vehicles having possibility of a collision (for example, refer to JP-A-2002-267750).
The monopulse type means one system performing an angle detection. That is, the monopulse type radar detects an angle of a detection object with respect to a front center direction of itself. In other words, in a case where the monopulse type radar is mounted in a front portion of the vehicle, the other vehicle in the front of the vehicle becomes the detection object, and the angle of the other vehicle in the front of the vehicle is detected by the monopulse type radar. On the other hand, in a case where the monopulse type radar is mounted in a rear portion of the vehicle, the other vehicle in a rear of the vehicle becomes the detection object, and the angle of the other vehicle in the rear of the vehicle is detected by the monopulse type radar.
Hereunder, there is additionally explained about the monopulse type by referring to FIG. 1 to FIG. 4.
As shown in FIG. 1, in a conventional monopulse type radar 1, there is provided a transmitting antenna 11, and receiving antennas 12-L, 12-Rare provided respectively in left and right sides of the transmitting antenna 11.
A transmission signal Ss is transmitted from the transmitting antenna 11.
This transmission signal Ss reflects in a detection object 2, and its reflection signal is received to the left receiving antenna 12-L as a reception signal Srl and received to the right receiving antenna 12-R as a reception signal Srr.
Thereupon, the monopulse type radar 1 calculates an angle θ of the detection object 2 by utilizing the reception signal Srl and the reception signal Srr by the monopulse type.
In this case, additionally the monopulse type can be divided broadly into a phase monopulse type and an amplitude monopulse type.
The phase monopulse type means the following system.
That is, as shown in FIG. 1, since a distance between the detection object 2 and the left receiving antenna 12-L differs from a distance between the detection object 2 and the right receiving antenna 12-R, it follows that a phase difference Δφ occurs between the reception signal Srl and the reception signal Srr. In this case, if a distance between the two receiving antennas 12-L, 12-R is described as d, the angle θ of the detection object 2 is shown like the following expression (1).Δφ=(2πd/λ)sin θ  (1)
In the expression (1), λ denotes a wavelength of the reception signals Srl, Srr.
Accordingly, the monopulse type radar 1 detects the phase difference Δφ between the reception signal Srl and the reception signal Srr, and calculates the angle θ of the detection object 2 on the basis of that phase difference Δφ and the expression (1).
The system like this is the phase monopulse type.
On the other hand, the amplitude monopulse type mentions the following system.
That is, a directivity DL of the left receiving antenna 12-L and a directivity Dr of the right receiving antenna 12-R are distributed like gain characteristics of FIG. 2 for instance. In this case, a signal intensity by an addition signal of the reception signal Srl of the left receiving antenna 12-L and the reception signal Srr of the right receiving antenna 12-R and a signal intensity by a difference signal between the reception signal Srl and the reception signal Srr become respectively like a curve Sadd and a curve Sdif shown by gain characteristics of FIG. 3, respectively. Additionally, a ratio between the signal intensities of both of the addition signal and the difference signal becomes like a curve R1 shown by a gain characteristic of FIG. 4.
Accordingly, the monopulse type radar 1 forms, by utilizing the reception signal Srl of the left receiving antenna 12-L and the reception signal Srr of the right receiving antenna 12-R, each of their addition signal and difference signal to thereby operate the ratio between the signal intensities of both of the addition signal and the difference signal, and calculates the angle θ of the detection object 2 by comparing an operation result thereof and a data of the gain characteristic of FIG. 4, which has been previously held.
The system like this is the amplitude monopulse type.
However, as shown in FIG. 5, in the conventional monopulse type radar 1, in such a case that, besides the detection object 2 exists in a front-left, a detection object 3 additionally exists in a front-right, an angle near 0 degree is detected as the angle θ. As a result, there has been a problem that, notwithstanding the fact that nothing actually exists near a front center direction, the conventional monopulse type radar 1 detects as if a detection object 4 exists near the front center direction, i.e., detects the detection object 4 which is merely a phantom.
Generation factors of this problem are as follows. That is, as shown in FIG. 5, in the left receiving antenna 12-L, it follows that a reflection signal of a transmission signal Ss2 in the detection object 2 is received as a reception signal Sr2l, and a reflection signal of a transmission signal Ss3 in the detection object 3 is received as a reception signal Sr3l. Similarly, in the right receiving antenna 12-R, it follows that the reflection signal of the transmission signal Ss2 in the detection object 2 is received as a reception signal Sr2r, and the reflection signal of the transmission signal Ss3 in the detection object 3 is received as a reception signal Sr3r. Accordingly, for the conventional monopulse type radar 1, by utilizing a mixed signal of the reception signal Sr2l and the reception signal Sr3l as a reception signal of the left receiving antenna 12-L, and a mixed signal of the reception signal Sr2r and the reception signal Sr3r as a reception signal of the right receiving antenna 12-R, respectively, the detection of the angle is performed. By this, it follows that the above-mentioned problem generates.
In this case, when a relative velocity v1 of the detection object 2 and a relative velocity v2 of the detection object 3, which are with respect to the monopulse type sensor 1, are different, due to Doppler effect it follows that frequencies of the reception signal Sr2l and the reception signal Sr3l are different respectively, and further it follows that frequencies of the reception signal Sr2r and the reception signal Sr3r are different respectively. Accordingly, if it is a monopulse type sensor capable of detecting a frequency (hereafter, referred to as a Doppler frequency), a phase and the like of a Doppler signal, e.g., a monopulse type sensor (refer to JP-A-2002-267750) in which a two-frequency CW system has been adopted, since a distinction between the reception signal Sr2l and the reception signal Sr3l can be made and further a distinction between the reception signal Sr2r and the reception signal Sr3r can be made, it is possible to solve the above-mentioned problem.
However, when the relative velocity v1 of the detection object 2 and the relative velocity v2 of the detection object 3 are the same, the Doppler frequency does not generate. That is, frequencies of the reception signal Sr2l and the reception signal Sr3l become the same, and further frequencies of the reception signal Sr2r and the reception signal Sr3r become the same. Accordingly, even if it is the monopulse type sensor in which the two-frequency CW system has been adopted, the distinction between the reception signal Sr2l and the reception signal Sr3l becomes impossible, and further the distinction between the reception signal Sr2r and the reception signal Sr3r becomes impossible. Thus, the above-mentioned problem still exists.
In other words, in the conventional monopulse type sensor, when the detection object near the front center direction has been detected, it is impossible to accurately specify whether that detection is a correct detection because the detection object in the front actually exists, or an error detection due to objects to the front-left and front-right of the radar.